Here's an example of how to use it (obviously a minimal case, you can customise your handler do do whatever you need):

    def mytrace(frame, event, arg):
        if event == "call":
            print("call", frame.f_code.co_name, frame.f_locals)
        elif event == "return":
            print("return", frame.f_code.co_name, arg)
        return mytrace

    import sys
    sys.settrace(mytrace)

    def three(z):
        return 3

    def two(y):
        return three("BBB") - 1

    def one():
        return two("AAA") - 1

    one()

    call one {}
    call two {'y': 'AAA'}
    call three {'z': 'BBB'}
    return three 3
    return two 2
    return one 1